Declining resistance resistor

ABSTRACT

A solid-state timer device which is comprised of a housing containing a mixture of either a powdered photosensitive metal salt or oxide with an inert conductor. Electrical leads are in contact with the mixture so as to pass a current therethrough. The electrical resistance of the mixture linearly declines with elapsed time at a fixed DC voltage input.

United States Patent Kenney 1 1 Jan. 25, 1972 [541 DECLINING RESISTANCERESISTOR 2,654,945 10/1953 Richardson et al. ..252/514 x Inventor: DavidA. y, Palos Verdes Estates, 2,486,341 10/1949 Stumbock ..252/514 X Cahf'Primary Examiner-Rodney D. Bennett, Jr. [73] Assignee: NorthropCorporation, Beverly Hills, Assistant Examiner-R Kinbel'g Calif.Attorney-Sokolski & Wohlgemuth and W. M. Graham [22] Filed: Nov. 24,1969 [57] ABSTRACT PP N04 879,205 A solid-state timer device which iscomprised of a housing 1 containing a mixture of either a powderedphotosensitive 52 us. 01 ..333/20 with Electrical leads 51 im. c1 ..H0lc7/10 are in Contact with the mixture as Pass 58 Field oiSearchm;..338/20; 252/506, 514 therethmugh- The electrical resistam mixturenearly declines with elapsed time at a fixed DC voltage input. [56]References Clted Claims, No Drawings UNITED STATES PATENTS 3,490,4401/1970 Mosier et a1. ..252/514 X PATENTEB m2 5 I972 INVEN'I'OR. DAV/D A.KENNEY SOKOLSK/ 8 WOHLGEMUTH ATTORNEYS DECLINING RESISTANCE RESISTORTiming devices having no moving parts are becoming increasinglyimportant. Uses for such timer devices include equipment time recordersto indicate the total time that the component has been operated. inconjunction with a simple circuit a timer of the type ofthis inventioncould be used to activate or deactivate a circuit or piece of equipmentafter a specific time. Additionally, such devices could be used ascompensators in instances where the output of an electrical device isknown to diminish at a specific rate with usage. A timer device couldthen be used to control a compensating circuit to step up electricalinput at an equal rate, thus resulting in a constant output.

The most prevalent form of timer available involves an electrochemicalcell. Utilizing Farraday's law, these prior timers generally deplate agiven amount of silver from an anode through electrolyte solution onto acathode surface. The timer serves to activate associated equipment whenthe deplating has been completed and the resistance greatly decreases atthat point. As a result, the amount of silver initially disposed on theanode must be carefully controlled so that repeatable results can beobtained from timer to timer as the amount of silver determines the timeperiod for the deplating at given current input.

The aforegoing cells have a disadvantage in that they contain a liquidelectrolyte which can leak and is often temperature and pressuresensitive. Further, the plating process must, as indicated, be completedto determine the total time. If one wishes to simply measure time priorto the complete deplating, then the cell must be removed from thecircuit in which it is used and expensive laboratory equipment isrequired to determine the amount of silver that has been deplated andcorrelate this to time elapsed. If it is desired to reuse such anelectrochemical cell then the current must be reversed and the silverdeplated must be replated onto the anode. This requires, of course,either removing the cell from the circuit in which it is used oralternatively changing the leads to the cell, both of which are notgenerally practicable.

Thus, an object of this invention is to provide a solid-state electrictiming device.

Another object of this invention is to provide a solid-state electronictiming device that can readily measure elapsed time.

A further object of this invention is to provide a solid-state electrictiming device which can operate at extremely low current and voltages.

Still a further object of this invention is to provide a solidstatetimer which is simple and inexpensive.

The above and other objects of this invention are accomplished by anovel electric timing device which comprises a housing which is eitherof a nonconductive material or a normally conductive composition whichis then insulated from the separate terminals and material containedtherein. In one embodiment, the housing can comprise one of theterminals. Within the housing is disposed a powdered mixture of aphotosensitive metal salt or oxide, with an inert conductor, such asgraphite. In contact with the material within the housing are twoterminals whereby leads from an electronic circuit can be attached tothe device so that current will flow through the powdered mixture. Aselectric current flows between the terminals the resistance of thedevice linearly decreases with elapsed time. By simply placing anohmmeter across the terminals one can determine the decrease inresistance of the element and thus determine the elapsed time. In someembodiments of the invention, the device will substantially remain atthe decreased resistance level after the current flowing therethrough isstopped. In other embodiments of the invention, the deice will return toa predetermined or original level of resistance when current hasstopped, and is thus ready for use once again. It is believed that theinvention will be better understood from the following detaileddescription and drawing in which:

The FIGURE is a cross-sectional view of a device of this invention.

It is believed that the operability of the invention is based on thereduction of metallic salt or oxide particles to the metallic ions suchas occurs in the photolysis action commonly used in photographicreproduction. In the instant invention it is believed that thisreduction is achieved through the use of electric current rather that bylight photons. For example, silver halides have a much greaterelectrical resistance than metallic silver. Thus, the conductivity of amixture containing silver halide particles will increase as the salt isreduced to metallic silver during the flow of current through thedevice.

It is believed that the inert conductive material such as graphite inthe mixture of the invention absorbs the gaseous ions liberated duringthe reduction of the salt, thus preventing recombination of the gaseousions with the resulting metallic lOl'lS.

Thus, it has been found that the amount of inert conductive materialpresent in the powdered mixture will affect the tendency of the deviceto return or retrogress toward the original resistance level aftercurrent flow is stopped. Thus, the more inert conductive powder presentto absorb or tie up liberated gaseous ions, the less will be thetendency for the device to retrogress. Further, it has been discovered,as will be explained in further detail, that some materials tend toretrogress more readily than others. For example, zinc oxide has agreater retrogression than the metal halides.

It has been observed for many compositions explored, that when thedevice does possess a retrogression ability, the first time the deviceis exercised over its maximum operating range, the initial resistancewill drop to a new value. Thereafter, it will return to this new valueshortly after stoppage of current flowing through the device.

Surprisingly, it has been discovered that the degree of compaction ofthe powdered mixture does not affect the retrogression characteristicsof the device. The degree of compaction mainly bears upon the initialresistance level. The greater the compaction, the lower the initialresistance. Satisfactory performance has been demonstrated with mixturescompacted as low as 3,000 p.s.i. The degree of compaction can range upto the point where no further compaction of the powder occurs. Withvarious tested compositions the upper composition pressure was about10,000 p.s.i.

The metal salts or oxides used are in a powder form. It has been foundthat the size of the powder particles does not have a significant effectupon the operation of the device of the invention. Typical particlesizes have ranged from 50 to 200 microns. Better packing and dispersalis achieved at the smaller particle sizes.

The preferred metal salts are the metal halides. Of the halides, themost preferred is the class of silver halides, particularly silverchloride, silver bromide and silver iodide. In addition to silverhalides, the results of this invention have also been observed utilizingother photosensitive halides such as the copper halides, including bothcuprous and cupric forms. Additional metal salts such as the oxalatesincluding ferric oxalate are contemplated as suitable normallyphotosensitive materials. Zinc oxide, which is also photosensitive, hasbeen found to be a suitable material to be combined with the inertconductive powder.

The normally photosensitive metal salts or oxides are combined with aninert conductive powder of about the same particle size range. Theconductive powder must obviously be comprised of a material whichconducts electricity yet will not react with the metal salt or oxide andthus is inert thereto. A most preferred material is graphite.Additionally, various compatible metal powders such as silver, silveroxide, and copper powders can be used.

As indicated, the mixture of powders can be merely compacted within thedevice of the invention. Alternatively, a small amount of an inertresinous binder such as an epoxy polymer or the like can be used. Thebinder will provide a solid element for the device when the resin iscured. In such an embodiment, the binder cannot be present in an amountsufficient to interfere with the conductivity of the powder mixture.

Thus, a very small amount of the binder is used sufficient to hold thepowders together in a solid state yet not interfere with the electricalproperties.

Turning to the FIGURE, there is seen a typical device made in accordwith this invention. The device 11 comprises an outer housing 13 whichcan, for example, be a tube of stainless steel or the like. So that theouter protective tube 11 is suitably insulated, a liner 15 of a resinmaterial such as an epoxy or the like can be utilized and can be bondedto the housing 13. Two terminals which are in the form of steel rods 17and 19 are concentrically disposed within each end of the cylindricalhousing 13. Each inward end of the rods of terminals 17 and 19preferably have a silver tinned facing 21 which serves to improve theconductivity. Between the two silver tinned faces 21 of the rods isdisposed a powdered mixture 23, having a composition as has beenpreviously described.

In this particular embodiment of the invention, the powder mixture 23can be compressed in situ within the tubular housing 13 by exertingpressure on either one or both of the rods or terminals 17 and 19,whereby they will serve as compression pistons. The rods are held incontact with the mixture by application of epoxy between the rods andthe housing. Altematively of course, the powder mixture 23 can becompacted to a desired degree and formed within a die press, or be mixedwith a binding agent and molded as a pellet. The electrical leads canalso be attached by embedding the ends of leads, enlarged to providesecure attachment and greater conducting area, during the compacting ormolding process. This assembly can then be encapsulated in a plastichousing, similar to standard construction techniques used for electricalresistors.

As has been indicated, the retrogression or return of the timing deviceof this invention to its initial resistance value upon the stoppage offlow of current is dependent upon the proportion of inert conductivematerial, such as graphite, present in the powdered mixture. The amountof conductive material that can be present can vary over a fairly widerange on a weight basis. However, since the photosensitive powders havewidely differing molecular weights, the ratios depend upon the givenmaterial chosen.

If the device is too rich in the inert conductive material, the metallicsalt particles will not experience sufficient electrical potential toreact so that a decline in resistance will be experienced. n the otherhand, the herein device will have a very severe drop in resistance if noinert conductive material is 1 present. The photosensitive powders arenot very conductive,

if at all. Thus, a maximum potential drop is experienced when noconductive material is present. This, of course, causes a rapid reactionand decline in resistance. From a practical standpoint, the hereindevice should require at least 0.10-milliamp current to operate it.Thus, there must be sufficient inert conductive material present torequire the aforegoing current. It has-been found that the device willoperate most successfully over a range of l to milliamps, indicating therelatively low current levels required. The more conductive materialpresent, the more current will be required. Thus, one could have enoughconductive material to require milliamps current. As a result, the upperlimit for the amount of conductive material is the point where nodecline in resistance is experienced when the device is subjected tocurrent. Thus, for example, with the silver halides, the weight ratio ofthe halide to graphite which gives the best results varies from 12:1 to24:]. For a copper halide, best results are obtained with the weightratio of halide to graphite of from 8:1 to 12:1, and for zinc oxide theratio of oxide to graphite, from 4:1 to 8:1. Since there are only twoingredients, it is a simple trial and error type of approach todetermine the most desirable ratio of ingredients for given materialsand desired end results.

The rate at which the resistance will decline across the compactedpowdered material 23 of the device of this invention is related closelyto the surface area of leads exposed to the material. For example, ifthe rods 17 and 19 have a Iii-inch diameter, the interface between therods and the material 23 will also, of 'course, be one-eighth inchdiameter in the embodiment shown. Such a device having a ifi-inchdiameter at the interface will have a rate of resistance declinesubstantially less than one that would have a diameter ofthree-sixteenths inch. A 3/ 16-inch diameter, for example, is equivalentto an increase in area of 225 percent.

For the aforegoing reason, it is not preferred, in instances where thecharacteristics of large area devices are important, to embed the leadsdirectly into the powder. The effective surface area for the currentpath traversing between the two leads would be greatly diminished fromthat shown in the FIGURE where the entire end of each lead contacts theequivalent end of the compacted powder material 23. it should be pointedout that the surface area of contact or effective surface area for thecurrent path to flow'between the electrodes, does not atfect thelinearity of the decline in resistance over a given time period. Thesurface area merely afi'ects the rapidity of such a decline. in someinstances it might be desirable to minimize the surface area contact soas to greatly slow down the decline in resistance. It should be notedthat one of the advantages of the instant device is its ability tofunction at low currents of less than 1 milliamp and low voltages.

In addition vto the embodiment shown, if one wanted to maximize thesurface area contact while not unduly enlarging the leads l7 and 19 perse, one could use enlarged conductive portions made out of material suchas graphite and the like that could be affixed to the ends of each leadin contact with an enlarged center section 23.

Additionally, one could make the container for the powdered material,one electrode having a very large surface area. The other electrodewould comprise a center pin concentrically disposed in the container andinsulated therefrom.

The retrogression characteristic of some of the embodiments of thisinvention can be important in certain end uses. However, in manyapplications it will make no difference whether or not the activematerial used in the timer device retrogresses or not, since one willnot be concerned with what occurs after current stops flowing throughthe device. Where the timer device serves to actuate the mechanism aftera certain time period, retrogression of the timer is of no moment as faras its performing that function. However, if the timer is removed afterit has served the timing function and allowed to reset itself byretrogression to its original or some other higher resistance value, itthen becomes a reusable item. In other applications where the timerserves to measure a continuous elapsed running time, retrogression isagain of no particular moment. An ohmmeter or like instrument wouldmerely be connected across the device and readings obtained at specifiedintervals to determine the resistance level and correlate this to theelapsed time. Thus, retrogression is most useful where it is desired toreuse a time that has performed a given function. Alternatively theability to provide a timer device that does not retrogress or willmaintain a fixed resistance level after operation is valuable in thatonce its timing function has stopped, one can always measure the elapsedtime that it had run by the resistance level at that point. It ispointed out that the device of the invention will in many instancesretrogress a particular percentage after each run. For example, a devicemight retrogress 10 percent from the resistance value reached. Thus,once this percentage is known, one can calibrate that device or type ofdevice taking into account the retrogression. Thus, though a device willretrogress, if such is always constant, the device can be operated asone that does not retrogress at all. It is believed that the inventionwill be further understood from the following detailed examples.

EXAMPLE I A device as shown in the FIGURE was used to perform the testsof this example. The inside diameter or diameter of the active powdermaterial was one-eighth inch, which was also the diameter of the twoleads. A mixture of silver bromide and graphite was used wherein theweight ratio of the bromide to graphite was 20: l. 0.32 grams of themixture was placed in the EXAMPLE [I The same mixture of powder as setforth in example I was used. However, only 30 percent of the quantity ofpowder used in example I was placed in the timer specimen housing to betested. The powder was compressed in the manner described in example Ito approximately the same initial resistance level as the sample inexample I. This was about 2,000

ohms. The required pressure to achieve this was approximately one-thirdthat used in example I. The reason for this is that the greater thequantity of powder between the leads or electrodes, the greater theinitial resistance of the device. Additionally, the initial resistancelevel of the device is affected by the degree of compaction of thepowder with the greater the compaction, the greater the initialresistance. Since only onethird the amount of powder was used,approximately only onechloride-graphite having an 20:] weight ratio, anda cupric chloride-graphite specimen having a ratio of 20:1

EXAMPLE V A zinc oxide and graphite specimen was made wherein the ratioof zinc oxide to graphite was 8:1. Activated charcoal, which has muchgreater absorptive properties than graphite, was used in an attempt toreduce the degree of retrogression associated with zinc oxide. The sizeand compression conditions were essentially the same as example I.

All of the specimens made in the foregoing examples were tested atambient temperature at approximately 5 volts DC. There were nocurrent-limiting resistors used during the test. Resistances weremeasured the first half hour, the first hour, and every hour on the hourthereafter for 8 hours, or until the specimen dropped below 50 percentof its initial value, at which time it was disconnected. Resistanceswere again measured 16 hours after specimens were disconnected todetermine retrogression characteristics. The following table summarizesthe results of the tests by indicating the initial resistance,resistance at the end of the test, and again at the end of 16 hours toshow retrogression.

TABLE Percent Final Approx. Initial End of drop Percent drop p.s.i. res,test res., After end drop test Specimen Composition compressed ohms ohms16 hrs. test final drop AgBr-Ex. I Silver bromide and graphite (20: l)10, 000 2, 265 966 1, 632 57. 3 28. O 49 AgBr-Ex. II do 10, 000 1, 893680 1, 343 64. 0 29. 1 63 AgBr-Ex II 3,000 2,072 1, 132 1, 630 45. 4 21.4 47 AgCl-G 10, 000 1, 903 910 1,576 52. 2 17. 2 33 AgI-G. 10, 000 2,274 952 1, 571 58. 1 31. 0 53 01101-6 10, 000 1, 469 1, 399 1, 393 4. 85. 2 1. 08 CuCla-(L. 10, 000 1, 387 992 1,053 28. 5 24. 1 .85 ZnO-G Zincoxide and graphite (8:1) ,000 2, 194 1, 794 2, 122 18. 2 3. 3 18 thirdof the pressure is needed to achieve the same initial resistance levelas the greater quantity of powder compressed to a greater degree, setforth in example I. As will eventually be sive and retrogressive thanthe partially compressed one of this example.

EXAMPLE III The same powdered mixture of example I was used. The weightof the powder was 0.72 grams or 225 percent more than the 0.32 grams ofexample I. However, in this example, the cross-sectional area of thecompressed powder, together with the pins, was 225 percent larger thanthat in example I. This resulted from the pins having a diameter ofthree-sixteenths inch, which was also the inside diameter of the device.The powder was compressed to approximately 2,000 ohms, which was thesame level of initial resistance of the device in example I. In orderthat the length of path through the powder between the electrodesremained the same as example I, 0.72 6

grams of powder was used. It will be seen that the percentage of declineper unit of time for this specimen with a large area was approximatelytwice that of the smaller area specimen in example I. It is believedthat this is due to there being approximately twice as many possibleconductive paths through the material. Retrogression characteristicswere about the same as the device of example I.

EXAMPLE IV A plurality of devices having the same dimension of andutilizing the same amount of powder were formed. These devices containeddifferent halide salts. These included a silver chloride-graphite devicehaving a ratio of chloride to graphite of 20:1; silver iodide-graphite,18:1 ratio; cuprous It is to be noted that the silver chloride andgraphite mixture actually performed about the same as the silver bromideand graphite one, since there was no adjustment for the difference inthe halide salts as to the ratio of the halide to the graphite betweenthe two. The silver iodide specimen was extremely responsive due to thelean mixture of graphite to silver iodide, yet the retrogressioncharacteristics were about the same as the silver bromide example. Thecuprous chloride specimen decline from approximately 2,000 to 1,469 ohmsprior to testing, without application of any current. However, a verysmall decline in resistance occurred during the test. This declinecontinued, but to a much lesser degree, after the specimen wasdisconnected. Likewise, the cupric chloride specimen declined fromapproximately 2,000 to 1,387 ohms prior to testing without applicationof current. The percentage of decline during the test was about halfthat of the silver bromide specimen of example I, which could be due tothe cupric chloride device being relatively rich in graphite.Retrogression, as can be seen, was relatively small. The zinc oxidenearly returned to its initial resistance after the 16 hours had elapsedfrom the end of the test. Thus, for repeatable applica- 0 tions, zincoxide will be the most desirable material to be used with the graphite.

Iclaim:

1. A timer device comprising:

a housing,

a powder mixture disposed in said housing of a normally photosensitivematerial selected from the group consisting of metal salts and metaloxides together with an inert conductive material, the ratio ofingredients being sufficient to provide a decrease in electricalresistance when a current is passed therethrough,

and at least two electrically conductive terminals in contact with andseparated by said powder mixture.

2. The device of claim 1 wherein said inert conductive material isselected from the group consisting of silver, silver oxide, graphite andcopper.

- 3. The device of claim 1 wherein said metal salts are selected fromthe group consisting of silver halides, copper halides and metaloxalates.

4. The device of claim 1 wherein said metal oxide is zinc oxide.

5. The device of claim 1 wherein the metal salt is a silver halide andthe inert conductive material is graphite.

6. The device of claim 5 wherein the weight ratio of silver halide tographite can vary from 12:] to 24:1.

7. The device of claim 1 wherein the metal salt is a copper halide andthe inert conductive material is graphite.

8. The device of claim 7 wherein the weight ratio of copper halide tographite can vary from 8:1 to 12:1.

ficient inert conductive material to require at least 0.10 milliamp ofcurrenttooperate said device.

*ssms

2. The device of claim 1 wherein said inert conductive material isselected from the group consisting of silver, silver oxide, graphite andcopper.
 3. The device of claim 1 wherein said metal salts are selectedfrom the group consisting of silver halides, copper halides and metaloxalates.
 4. The device of claim 1 wherein said metal oxide is zincoxide.
 5. The device of claim 1 wherein the metal salt is a silverhalide and the inert conductive material is graphite.
 6. The device ofclaim 5 wherein the weight ratio of silver halide to graphite can varyfrom 12:1 to 24:1.
 7. The device of claim 1 wherein the metal salt is acopper halide and the inert conductive material is graphite.
 8. Thedevice of claim 7 wherein the weight ratio of copper halide to graphitecan vary from 8:1 to 12:1.
 9. The device of claim 1 wherein the metaloxide is zinc oxide and the inert conductive material is graphite. 10.The device of claim 9 wherein the weight ratio of zinc oxide to graphitecan vary from 4:1 to 8:1.
 11. The device of claim 1 wherein said powdermixture is pressed to a compact mass.
 12. The device of claim 1 whereinsaid powder mixture is disposed in an inert solid binder.
 13. The deviceof claim 1 wherein the mixture contains sufficient inert conductivematerial to require at least 0.10 milliamp of current to operate saiddevice.